Directional drilling is now commonly practiced in petroleum related drilling activity. The control of the course of the progressing well bore has historically been an expensive and time consuming activity. Extensive effort has been made to provide apparatus for use in the bottom hole assembly of the drill string that can be activated to change the course of the progressing well bore, then convert back to optimum drilling arrangements. That objective has experienced very limited success.
In many earth formations, it is convenient and economical to drill with down hole drilling motors. In many of those cases it is necessary to exercise control of the course of the progressing well bore while drilling. The need for directional control is usually brief. Once correction is made in the well bore course, optimum drilling conditions can usually be restored for an extended period of time.
The drilling motor usually has a flexible drive shaft between the motor and the output shaft that drives the drill bit. Further, the output shaft that drives the drill bit is supported on a rugged bearing arrangement with a rotational axis that coincides with the well bore center line. When bent motor housings are used to deflect a well bore while drilling, the bend is usually between the motor and the bearing assembly supporting the motor output shaft. Usually, the flexing ability of the flexible drive shaft can accommodate the bend without modification. The bent body is often used with a non-rotating drill string for deflecting the well bore, then, it is rotated to produce a straight hole. The stresses produced by rotating a bent body can often be accommodated by the bottom hole assembly for a short time. As a long term matter, there is reduced confidence in the reliability of the bottom hole assembly.
There are deflecting apparatus for selectively bending the motor housing for directional work, then straightening the housing for straight hole drilling. Activation of such equipment usually involves communication from the surface. There is usually some uncertainty about the apparatus receiving and responding to such communication. The return to straight drilling mode may be attended by the same uncertainty.
The common drilling motor is powered by drilling fluid and starts at the onset of flow of drilling fluid. It can stall briefly without consequent damage. While it is stalled, some fluid leaks through. The drilling motor is capable of considerable torque. If the stall factor is suddenly released, the pressure drop through the motor may change well over two hundred psi. These factors make the drilling motor an ideal power source for activating drill string deflecting apparatus.